1. Field of the Invention
The present invention relates to an ink jet apparatus that prints by ejecting ink droplets under pressure from nozzles.
2. Description of the Related Art
Traditional impact printers are now being replaced with non-impact printers, and the market of the non-impact printers is being expanded. One known kind of non-impact printers is an ink jet printer simple in principle and that can easily effect multi-scale or color printing. Of all of the types of ink jet printers, a drop-on-demand type ink jet printer capable of jetting ink droplets at a required time during printing has rapidly spread owing to its good jetting efficiency and its low running cost.
Typical examples of such drop-on-demand type ink jet printers, are a Kaiser type disclosed in Japanese Patent Publication No. Sho 53-12138 and a thermal jet type disclosed in Japanese Patent Publication No. Sho 61-59914, for example. However, the former is hard to reduce in size, and the latter is required to have a high heat resistance of ink because the ink undergoes a high temperature. Thus, both types have very severe problems in application.
To solve the above problems, there has been a newly proposed shear mode type disclosed in U.S. Pat. No. 4,887,100, for example.
FIG. 16 shows a shear mode type ink jet apparatus 1 in the prior art. As shown in FIG. 16, the ink jet apparatus 1 is constructed of a piezoelectric ceramics plate 2, a cover plate 10, a nozzle plate 14, and a substrate 41.
The piezoelectric ceramics plate 2 is provided with a plurality of grooves 3 by grinding with use of a diamond blade or the like. Accordingly, a plurality of side walls 6 extend along the grooves 3 in such a manner that each side wall 6 is formed between adjacent ones of the grooves 3. Each side wall 6 is polarized in a direction indicated by an arrow 5. All the grooves 3 have the same depth, and they are parallel to each other. The depth of each groove 3 is gradually reduced as it approaches a rear end surface 15 of the piezoelectric ceramics plate 2 to form a shallow groove 7 near the rear end surface 15. A pair of metal electrodes 8 are formed on opposed side surfaces of each groove 3 at an upper half portion thereof by sputtering or the like. Further, a metal electrode 9 is formed on opposed side surfaces and a bottom surface of each shallow groove 7 by sputtering or the like. The pair of metal electrodes 8 formed on the opposed side surfaces of each groove 3 are connected with the metal electrode 9 formed on the opposed side surfaces and the bottom surface of the corresponding shallow groove 7 contiguous to the groove 3.
The cover plate 10 is formed of a ceramics material, a resin material, etc. The cover plate 10 is provided with an ink inlet hole 16 and a manifold 18 communicating with the ink inlet hole 16 by grinding, cutting, etc. The lower surface of the cover plate 10, on which the manifold 18 is formed, is bonded to the upper surface of the piezoelectric ceramics plate 2 on which the grooves 3 are formed by an epoxy adhesive 20 (see FIG. 18). Accordingly, a plurality of individual ink chambers 4 functioning as ink channels (see FIG. 18) are defined by the grooves 3 of the piezoelectric ceramics plate 2 and the lower surface of the cover plate 10 to be transversely equally spaced from each other. As shown in FIG. 18, each ink chamber 4 is rectangular in vertical section, and it is filled with ink in operation.
As shown in FIG. 16, the nozzle plate 14 is bonded to the front end surface of the assembly of the piezoelectric ceramics plate 2 and the cover plate 10. The nozzle plate 14 is provided with a plurality of nozzles 12 at laterally spaced positions corresponding to the front end positions of the ink chambers 4. The nozzle plate 14 is formed of a plastic material such as polyalkylene terephthalate (e.g., polyethylene terephthalate), polyimide, polyetherimide, polyetherketone, polyethersulfone, polycarbonate, or cellulose acetate.
The substrate 41 is bonded to the lower surface of the piezoelectric ceramics plate 2 on the opposite side of the cover plate 10 by an adhesive such as an epoxy adhesive. A plurality of individual conductor film patterns 42 are formed on the substrate 41 at transversely spaced positions corresponding to the rear end positions of the ink chambers 4. Each conductor film pattern 42 is connected through a conductor wire 43 to the metal electrode 9 formed on the bottom surface of the shallow groove 7 in the corresponding ink chamber 4 by wire bonding.
FIG. 17 shows a schematic diagram of a control section for controlling the ink jet apparatus 1. As shown in FIG. 17, the conductor film patterns 42 formed on the substrate 41 are individually connected to an LSI chip 51. Also connected to the LSI chip 51 are a clock line 52, a data line 53, a voltage line 54, and a ground line 55. The LSI chip 51 determines from which nozzle 12 the ink droplets are to be jetted according to data appearing on the data line 53 on the basis of continuous clock pulses supplied from the clock line 52. Then, according to the result of determination, the LSI chip 51 applies a voltage V of the voltage line 54 to the conductor film pattern 42 connected to the metal electrode 8 in the ink chamber 4 to be driven. Further, the LSI chip 51 applies a zero volt of the ground line 55 to the other conductor film patterns 42 connected to the metal electrodes 8 in the other ink chambers 4 not to be driven.
The operation of the ink jet apparatus 1 is described with reference to FIGS. 18 and 19. When the LSI chip 51 determines that the ink droplets are to be jetted from the nozzle 12 corresponding to the ink chamber 4b as one of the ink chambers 4 of the ink jet apparatus 1 according to given data, a positive driving voltage V is applied to the metal electrodes 8e and 8f and the metal electrodes 8d and 8g are grounded. As shown in FIG. 19, a driving electric field in a direction indicated by an arrow 13b is generated in the side wall 6b, and a driving electric field in a direction indicated by an arrow 13c is generated in the side wall 6c. As the directions indicated by the arrows 13b and 13c of the driving electric fields are perpendicular to the direction indicated by the arrow 5 of polarization of the piezoelectric ceramics plate 2, the side walls 6b and 6c are rapidly deformed inwardly of the ink chamber 4b by a piezoelectric thickness shear effect. This deformation of the side walls 6b and 6c reduces the volume of the ink chamber 4b to rapidly increase the pressure of the ink filled in the ink chamber 4b and thereby generate a pressure wave. As a result, the ink droplets are jetted from the nozzle 12 (see FIG. 19) communicating with the ink chamber 4b.
When the application of the driving voltage V is stopped, the side walls 6b and 6c gradually restore their original positions before deformation (see FIG. 18), and the pressure of the ink contained in the ink chamber 4b is therefore gradually decreased. Then, additional ink is supplied from an ink tank (not shown) through the ink inlet hole 16 (see FIG. 16) and the manifold 18 (see FIG. 16) into the ink chamber 4b.
Referring to FIG. 14 for explanatory purposes, which is a sectional side view of the ink jet apparatus according to the invention, when the pressure in each ink chamber 4 is increased to jet the ink droplets, the ink is forced from the corresponding nozzle 12 simultaneously, the ink reversely flows from the manifold 18 into the ink inlet hole 15. As a result, the pressure near the manifold 18 is rapidly reduced to generate a negative pressure wave. When this negative pressure wave reaches the nozzle 12, the ink jet from the nozzle 12 is stopped. The shorter the distance y between the front side surface of the manifold 18 and the inner surface of the nozzle plate 14, the shorter the time of reach of the negative pressure wave to the nozzle 12. Accordingly, when the distance y is reduced, the ink jet from the nozzle 12 is quickly stopped to result in a reduction in volume of ink droplets, causing a deterioration in print quality. On the other hand, when the distance y is largely increased, to cope with the above problem, the distance x between the front side surface of the manifold 18 and the rear end surface of each ink chamber 4 becomes very small. Accordingly, the ink flow from the manifold 18 into each ink chamber 4 becomes difficult, so that a necessary amount of ink cannot be supplied to each ink chamber 4. As a result, the volume of ink droplets is reduced to cause deterioration in print quality.